The present invention relates broadly to an optical amplifier arrangement for use in an optical path along which a bi-directional optical signal is travelling, and to an optical network incorporating such an optical amplifier arrangement.
FIG. 1 illustrates how a bi-directional amplifier arrangement 10 may be implemented. The bi-directional amplifier arrangement 10 comprises two unidirectional amplifiers 12, 14, 3-port optical circulators 18, 20 are used as optical junction elements at either end of the amplifier arrangement 10. Isolators e.g. 16 impart additionally the directionality of the bi-directional amplifier arrangement 10.
In the bi-directional amplifier arrangement 10 no protection against the optical field undergoing dual reflective events 22, 24 after the optical circulators 18, 20 respectively is provided. Therefore, the optical field can potentially fully circulate within the amplifier arrangement 10. In the case where the re-circulation pass gain experienced is higher than the loss from the dual reflective events 22, 24, lasing or instability will occur which can severely impair the performance of the amplifier arrangement 10.
FIG. 2 provides an illustration of how this problem may be overcome. In particular, the arrangement 50 utilises filter elements 32, 34 in the respective optical paths 26, 28 of the amplifier arrangement 50, whereby only a preselected wavelength e.g. (xcex1 in the case of filter 32 and e.g. xcex2 in the case of filter 34) can propagate along the respective optical paths 26, 28. However, in order to prevent lasing such an amplifier arrangement must be configured such that no wavelength can propagate along both optical paths, which means that the direction of amplification of the resulting amplifier arrangement is wavelength dependent. Therefore, this solution is not well suited for use in optical networks in which the propagation direction of optical signals within the optical network is not wavelength dependent, i.e. in optical networks that provide a higher flexibility in the bi-directional data transmission.
Furthermore, the filter elements 32, 34 may cause degradation of the optical signals passing through them due to, e.g. their dispersion characteristics. In an optical network or transmission link employing a plurality of amplifiers using the arrangement 50, this degradation may accumulate, resulting in the signal becoming unrecoverable at the receiver.
At least preferred embodiments of the present invention seek to provide an alternative bi-directional amplification arrangement.
In accordance with a first aspect of the present invention there is provided a bi-directional optical amplifier arrangement for use in an optical path along which a bi-directional optical signal is travelling, the arrangement comprising at least two amplifier units disposed to amplify, in use, in different propagation directions, at least two propagation direction dependent optical junction elements at least two optical paths disposed in parallel between the junction elements, wherein each optical paths comprises one of the amplifier units; and wherein gain settings of the amplifier units are chosen in a manner such that, in use, a total gain experienced by an optical signal re-circulating along both optical paths is less than unity.
The amplifier arrangement may further comprise at least one optical isolator element disposed along each of the optical paths, for enhancing the directionality of the respective optical paths.
Advantageously, the junction elements each comprise an optical circulator. The optical circulator is preferably a blocking optical circulator.
In a preferred embodiment, the gain settings of the optical amplifier units are chosen such that the sum of the gain settings is less than the maximum loss associated with reflection directly at the junction element.
The respective gain settings of the amplifier units may be symmetric or asymmetric.
In one embodiment, the amplifier arrangement further comprises at least one filter element disposed along each of the optical paths for selectively amplifying components of the bi-directional optical signal. The filter elements in the respective paths may be incorporated in the respective amplifier units.
In accordance with a second aspect of the present invention there is provided an optical network comprising an optical amplifier arrangement, in accordance with the first aspect of the present invention.
Preferably, the optical network comprises at least one pair of network nodes, wherein the amplifier arrangement is disposed between the network nodes.
The optical network may comprise a bi-directional WDM network. Preferably, the WDM network is a dense WDM network.